This application claims the benefit of Japanese Application No. 10-010929, filed in Japan on Jan. 23, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a synthetic silica glass, and more particularly, to a synthetic silica glass that is used as an optical member in the wavelength range of 400 nm or less, and its manufacturing method.
2. Discussion of the Related Art
Currently, an exposure device called a stepper is used in photo-lithographic technology in which a very small pattern for an integrated circuit is exposed and transcribed onto a substrate, such as silicon wafer. Due to a recent demand towards higher integration of LSI, the light source of this stepper is becoming shorter and shorter; from the g-line (436 nm) to the i-line (365 mn), and further to KrF excimer lasers (248 nm) and ArF excimer lasers (193 nm). Existing optical glass has a low optical transmittance so that it cannot be used in the wavelength range that is shorter than the i-line. Therefore, in general, synthetic silica glass has been used for the optical member for the illumination system and the projection lens for the stepper.
However, even with the synthetic silica glass, a deterioration in optical performance, such as reduction in transmittance, occurs after the synthetic silica glass is used under high-powered ultraviolet rays or excimer laser for an extended period of time.
The cause of the deterioration in transmittance is considered to be structural defects, such as the Exe2x80x2 center, NBOHC (Non-Bridging Oxygen Hole Center), and the like, generated in the silica glass due to irradiation by ultraviolet rays.
In the past, in order to improve the durability of the silica glass against ultraviolet rays, a technique in which silica glass is heated in a hydrogen atmosphere (Japanese Laid-Open Unexamined Patent Publication No. 01-201664) and a technique in which hydrogen molecules are doped into the silica glass (Japanese Laid-Open Unexamined Patent Publication No. 03-109233) have been proposed. These methods attempt to repair the structural defects by hydrogen to improve the durability against ultraviolet rays.
However, in these methods using hydrogen, although portions of the glass having these structural defects may temporarily be repaired, the precursors of the structural defects themselves cannot be removed. Examples of such precursors are Sixe2x80x94Oxe2x80x94Oxe2x80x94Si, Sixe2x80x94Si, and Sixe2x80x94Cl bonding structures, which are different from the bonding structure of silica (Sixe2x80x94Oxe2x80x94Si), and distorted bonding structures in the Sixe2x80x94Oxe2x80x94Si bonds. These precursors in silica glass are easily converted into structural defects under ultraviolet irradiation even after the structural defects are cured by hydrogen.
Thus, it is desirable to develop a synthetic silica glass and a manufacturing method for the synthetic silica glass, which has a superior durability against ultraviolet rays by directly reducing the precursors of the structural defects.
Accordingly, the present invention is directed to a synthetic silica glass and its manufacturing method that substantially obviate the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a synthetic silica glass, which has less structural defects and thereby has a stable structure, and its manufacturing method.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a method for manufacturing a synthetic silica glass, the method including the steps of maintaining a silica glass member, which is formed using a flame hydrolysis method and having an OH group concentration of about 500 ppm to about 1300 ppm and a hydrogen molecule concentration of about 1xc3x971016 molecules/cm3 or greater, at a predetermined holding temperature for a predetermined period of time so as to substantially relax the structure of the silica glass member; thereafter, cooling the silica glass member to a first predetermined temperature at a cooling rate of about 10 K/hour or less; thereafter, cooling the silica glass member to a second predetermined temperature at a cooling rate of about 1 K/hour or less; and thereafter, cooling the silica glass member to a third predetermined temperature at a cooling rate of about 10 K/hour or less.
In another aspect, the present invention provides a method for manufacturing a silica glass having a reduced structure determination temperature, the method including the steps of annealing a silica glass member having a glass state corresponding to a predetermined structure determination temperature, the annealing being performed at a first temperature lower than the predetermined structure determination temperature for a time period equal to or longer than a relaxation time of the silica glass member at the first temperature; and thereafter, cooling the silica glass member at such a cooling rate that until the silica glass member reaches a second temperature, the thermodynamical structural relaxation process of the silica glass member substantially follow the temperature changes, the cooling rate being such that below about the second temperature, the thermodynamical structural relaxation process of the silica glass member no longer substantially follow the temperature changes, thereby super-cooling the silica glass member to produce a silica glass having a structure determination temperature lower than the predetermined structure determination temperature.
In a further aspect, the present invention provides a method for manufacturing a silica glass having a reduced structure determination temperature, the method including the steps of forming a silica sample having a glass state corresponding to a predetermined structure determination temperature; annealing the silica sample at a first temperature lower than the predetermined structure determination temperature, the annealing being performed for a time period equal to or longer than a relaxation time of the silica sample at the first temperature; and thereafter, cooling the silica sample at such a cooling rate that until the silica sample reaches a second temperature, the cooling process is substantially quasistatic, the cooling rate being such that below about the second temperature, the cooling process no longer is substantially quasistatic, thereby super-cooling the silica sample to produce a silica glass having a structure determination temperature lower than the predetermined structure determination temperature.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.